The present invention relates to the field of image processing, and more particularly to digitally storing one or more selected scan lines from a composite video signal in real time.
Typically, the first step in image processing is acquisition of the image by a video camera viewing the scene of interest that provides a composite video signal. A composite video signal, such as an NTSC (National Television Standards Committee) signal, consists of successive frames having a fixed number of lines. Each line is composed of a fixed number of pixels (pixel elements) where each pixel has an associated intensity.
Referring to FIG. 7, which illustrates a single horizontal line of an NTSC composite video signal in graphical form, images are formed from a continuous analog signal representing a line-by-line scanning of the picture brightness along each line. At the beginning of each scan line, timing information is inserted to provide triggering and a sufficient interval for the camera setup. A horizontal sync pulse follows the "front porch" and identifies the beginning of a line of the video signal. A vertical sync pulse, not shown, is provided to identify the beginning of every frame.
The composite video signal is transformed into an appropriate digital representation consisting of one or more frames each represented by an n.times.m array of elements.
Traditional approaches in image processing employ a "frame grabber" which consists of circuitry that converts an entire two dimensional video image into a corresponding digitized array which can then be stored in a computer memory. Each element of the array corresponds to a pixel of a horizontal line of the image. For every pixel there must exist an associated location in memory which stores a value corresponding to scene intensity (grey level) for that particular pixel. For example, the number of memory locations (bytes) necessary to store just one frame of a conventional 525-line NTSC television image having a resolution of 512 pixels per horizontal line is 268,800 (525.times.512). The second image processing step involves processing the acquired data. Due to the size of the array necessary to store one frame (525 by 512 elements), even the simplest of operations, such as thresholding to convert the video signal to a binary image, is time consuming. More elaborate operations simply cannot be done in real-time, and certainly not before the next image arrives at the NTSC rate of 60 frames/second.
Some applications, however, do not require all of the data captured by a conventional frame grabber. A good example of such an application is video motion detection as employed in automatic security systems for purposes of identifying the presence of an intruder in a scene under surveillance. Experiments have shown that in such physical security scenarios, it is possible to detect motion by examining only one horizontal line which cuts through the region of interest in the scene. Several horizontal lines equally spaced throughout the scene can be used to obtain full screen coverage, without the need to "grab" the entire frame. The image processing needs are thus greatly reduced, and in most cases can be performed during the wait period between lines of interest when the acquisition system is idle, i.e., in real time. As an example, in an application where capture of only a single line is sufficient, the memory size requirement is reduced to half a kilobyte, and approximately 16.4 milliseconds are available for processing before the next line must be digitized.
Accordingly, an object of the present invention is to provide a video line digitizer that can reconfigure itself to identify and store different predetermined lines in response to the outcomes of one or more decisions. Another object of the present invention is to provide a video line digitizer that requires a relatively small memory capacity with respect to a frame grabber that stores an entire video frame. A further object of the present invention is to provide a video line digitizer that is small enough to fit on a mobile platform such as a robot. Another object of the invention is to provide an video line grabber that has a low power consumption. Yet another object of the present invention is to provide a video line digitizer that identifies and stores predetermined lines of successive video frames in real time and then provides an output functionally related to differences between those lines.